To cement a string of casing into a well bore, a work string with one or more cementing plugs disposed at a lower end thereof extends into and connects to a casing running tool that suspends the casing string to be cemented. The work string runs the casing string into the well bore to the desired depth. Then a cement slurry is pumped downwardly through the work string, the casing string, and upwardly into the annular space formed between the casing string and the walls of the well bore. Upon setting, the cement bonds the casing string to the walls of the well bore and restricts fluid movement between formations penetrated by the well bore.
When the casing string is run into the well bore, the casing string fills with drilling fluid or other fluid in the well bore. To reduce contamination of the cement slurry at the interface with the drilling fluid, one of the cementing plugs is released from the work string and pumped ahead of the cement slurry. Specifically, a dart or other releasing device is dropped down the work string ahead of a batch of cement and the dart lands in a seat in one of the cementing plugs. The pressure behind the dart causes the cementing plug to be released as the cement pushes the plug down. The cementing plug thereby maintains a separation between the cement slurry and the drilling fluid until the cementing plug lands on a float collar or float shoe attached to the bottom end of the casing string. The cementing plug also sealingly engages the inner surfaces of the casing string to wipe the drilling fluid from the walls of the casing string ahead of the cement slurry.
The cementing plug that precedes the cement slurry and separates it from the drilling fluid is referred to herein as the “bottom cementing plug.” When the required quantity of cement slurry has been pumped through the work string, a second cementing plug, referred to herein as the “top cementing plug”, is released from the work string to separate the cement slurry from additional drilling fluid or other fluid used to displace the cement slurry through the casing string. Specifically, a wiper dart is launched from the surface to follow the cement, thereby wiping the cement from the walls of the work string, then landing in a releasing sleeve of the top cementing plug thereby releasing the top cementing plug to be pumped down the casing string.
When the bottom cementing plug lands on the float collar or float shoe attached to the bottom of the casing string, a bypass mechanism in the bottom cementing plug is actuated which allows the cement slurry to proceed through the bottom cementing plug, through the float collar or float shoe and upwardly into the well bore annulus between the casing string and the well bore wall. The design of the top cementing plug is such that when it lands on the bottom cementing plug, it shuts off fluid flow through the plugs, which prevents the displacement fluid from entering the well bore annulus.
During run in of the casing string into the well bore, before the cementing operation begins, the one or more cementing plugs allow drilling fluid or other well bore fluid to flow upwardly into an annular space formed between the casing string and the work string, but the cementing plugs prevent flow downwardly out of the annular space. Therefore, the annular space holds a trapped volume of fluid. Pressure can build in this trapped volume of fluid, and unless such pressure is relieved, it will exert a downward force on the one or more cementing plugs that could cause the plugs to be damaged and/or release from the work string prematurely.
Heretofore, one or more equalizer valves have been included in the work string to relieve the pressure in the trapped volume of fluid back into the work string. However, such equalizer valves may not relieve the pressure quickly enough to prevent damage to the cementing plugs when the trapped volume of fluid is large. Therefore, a need exists for an improved cementing assembly that will reliably relieve fluid pressure at a sufficient rate to prevent damage to and/or release of the one or more cementing plugs regardless of the volume of fluid trapped in the annular space between the casing string and the work string.